In power generation and transmission, electrical generator coils or windings are used to produce alternating electric current. To effect the more economical transmission of produced electric current, polyphase transmission systems have been developed. Conventionally, most polyphase power transmission is effected in the form of three-phase power transmission where three alternating currents are produced. Three-phase electrical transmission systems are typically based on a three conductor construction, each conductor used to transfer individual alternating currents which are produced. The alternating currents are generated to reach respective instantaneous peak values at different times, with the second and third currents being delayed respectively by one-third and two-thirds of the current cycle time.
In electrical distribution equipment used to transmit high amperature electrical power from the grid to a load area, the generated polyphase power is supplied by way of electrically insulated stranded conductors or cables when installed near electrical grounds. As the amperature electric current to be delivered increases to higher values, the three conductor constructions become less economical and feasible to use due to the damaging mutual heating of the conductor insulation. Therefore, multiple single conductor cables spaced and strategically positioned apart, are provided for each separate phase as a means to effect equal current sharing, and prevent current varying along any one cable which could otherwise result in overheating. Where such multiple cable delivery systems are provided, it is necessary that the cables are arranged in a parallel format for each phase in a well spaced and oriented manner to avoid electrical current, as well as magnetic field imbalance. The specific cable spacing and orientation varies on an installation-by-installation basis, depending on a number of limitations on current carrying factors, such as the current amperature, cable size, as well as thickness and/or quality of cable conductor insulation.
In an effort to ensure optimal electrical power cable spacing and orientation, various cable bus systems have been developed to provide both mechanical protection and ensure the desired positioning of electrical cables. A prior art cable bus system 6 of the type sold under the name Superior Cable Bus™ by Superior Tray Inc. of British Columbia, Canada and MP Husky™ Cable Bus by MP Husky in Greenville, S.C., are shown in FIG. 1. In such conventional cable bus system 6, a number of electrical cables 12 are housed within a metal enclosure 8. The metal enclosure 8 is provided with parallel spaced C-channel sidewalls 16a,16b which are joined along the top and bottom edges respectively by a ventilated top panel 18 and a bottom panel 20 to define an interior raceway 22.
Along each side of the raceway 22 opposing pairs of C-shaped rails 23 are provided at three foot intervals for each receiving and retaining therein in a series of split blocks 24 which must be locked together using bolts on each side. The split blocks 24 are used to secure the cables 12 in place within the raceway 22, in a sandwiched arrangement. Each split block 24 is formed as a series of chocks 26a,26b,26c which are approximately one inch in thickness. The chocks 26 are provided with a series of pre-drilled holes 28 which are centered along their abutting edges. The holes 28 are sized to receive respective cables 12 therein. The holes 28 are spaced and positioned across the block 24 at multiple levels to receive and support a number of electric cables 12 in a parallel relationship therein.
The applicant has appreciated that various limitations exist with prior art split block cable bus systems 6. In particular, the use of split chocks 26a,26b,26c to support and position electrical cables 12 are both cumbersome and time consuming. In certain cases physical space constraints may prevent the installation of bolts necessary to lock the blocks together. Because it is not technically allowable or feasible to splice the parallel cables in each polyphase used in high amperature cable transmissions systems, it is necessary to physically draw full circuit lengths of cable in each layer of the electrical cables 12 successively over each chock 26a,26b at the site of installation. Because of the longitudinal length spacing between the sets of split blocks and the varying surface contour of each split block 24, this in turn necessitates the use of labour intensive cable rollers to avoid cable damage, increasing both the time and cost of installation.
In addition, if the outside diameter of the cable insulation layer 13 varies relative to the diameter of preformed holes 28 as a result of manufacturing tolerances, the holes 28 formed in the cable support blocks 24 could be either too loose or too tight for proper cable mounting. The correction of hole sizes is both difficult and costly, if deemed possible, and therefore requires the manufacturing and replacement of the split blocks.
Another disadvantage is that the cables used are normally heavier, larger sized conductors with relatively thin insulation which can be readily ruptured and fail at the three foot interval support locations, as a result of commonly occurring and damaging electrical system fault forces.
Further, the positioning of split blocks 24 at approximate three foot intervals results in the formation of sectionalized compartments along the length of the raceway 22. If ventilation openings in the top panel 18 are inadvertently covered by debris or the like, this disadvantageously may result in localized cable hot spots along the raceway 22 as a result of blockage of required airflow.
More problematic however, if cables 12 at a lower level require removal or replacement as a result of damage or failure, with prior art systems it is necessary to first completely remove and thereafter reinstall all overlying chock blocks 26c,26d and the upper cable layers from the raceway 22 at significantly increased time, cost and difficulty.
Another disadvantage of the prior art of cable bus systems are typically installed above ground level in order to ensure that free flowing air passing through the vented covers dissipates undesirable heat generated by cables away from the enclosure. Heat trapped within the vicinity of the high amperage cables will cause the ambient temperature to rise, causing premature failure of the cable installation or necessitating the substantial derating of the allowable cable amperature, which can become cost prohibitive.
Power cables of varying levels of amperage are commonly installed underground by either underground conduits encased within poured concrete, or alternately directly buried. Due to the slow rates of heat transfer away from the heat generating cables due to a lack of airflow, the amperage of cables installed in such mediums are restricted both technically and by industry standards to approximately one half of that allowable for cables which are continuously cooled by air movement. When a power cable circuit is conventionally installed partially underground with the balance of the circuit installed where there is open airflow, the allowable amperage for the cables in the total circuit length is restricted to the lower underground amperage values resulting in uneconomical installation practices.